1. Field of the Invention
The invention relates to a process and apparatus for waste water purification by evaporation of the water to be purified in an entrainment gas current and removal of the residue which is thereby formed.
2. Description of the Prior Art
The primary problems in the purification of waste water are caused by dissolved impurities and pollutants which, although not dissolved in the waste water, are not sedimentary on account of their fineness. Of special importance are industrial waste waters, for which additional purification stages such as chemical precipitations, neutralization of impurities or detoxification by the addition of oxidation agents and the use of ion exchangers, are necessary even before general purification in clarification installations. In most cases, metallic compounds or salts also occur during the purification of such industrial waste waters, the recovery or recycling of which metallic compound or salts is of economic advantage. A qualitatively thorough cleaning of the water up to the purity of distilled water is the objective such processes which include the treatment of brackish water and desalination of sea water. The latter is of special interest in countries which do not have abundant groundwater or rainwater.
A thorough purification of water can be achieved by distillation. The water is heated in an evaporator up to saturation temperature, the steam formed is extracted and condensed in a cooled condenser. When there is complete evaporation, those substances which cannot be evaporated remain in the evaporator as a solid residue. With such an open distillation process, the water is generally not completely distilled, but a liquid fraction very high in salts is extracted from the sump of the distillation column. In this manner, the open distillation process can be carried out continuously, however, the high-salt residue is generally not reusable in the form in which it is extracted from the distillation apparatus and the residue requires further treatment.
Pervaporation is also a known separation process (See, for example, Chemie-Ingenieur-Technik, 1982, No. 3, pp. 229-224, especially p. 232). In this process, fluid to be purified is conducted along the primary side of a membrane to the secondary side of which the components permeating the membrane are transferred in the vapor stage and transported away by a carrier gas. In this process, a high degree of selectivity is achieved in the separation of dissolved components. The substances which do not permeate the membrane remain in the residue fraction on the primary side of the membrane and cannot be separated from it without additional measures.